Audio signals, like speech or music, are encoded for example to enable efficient transmission or storage of the audio signals.
Audio encoders and decoders (also known as codecs) are used to represent audio based signals, such as music and ambient sounds (which in speech coding terms can be called background noise).
An audio codec can also be configured to operate with varying bit rates. At lower bit rates, such an audio codec may be optimized to work with speech signals at a coding rate equivalent to a pure speech codec. At higher bit rates, the audio codec may code any signal including music, background noise and speech, with higher quality and performance. A variable-rate audio codec can also implement an embedded scalable coding structure and bitstream, where additional bits (a specific amount of bits is often referred to as a layer) improve the coding upon lower rates, and where the bitstream of a higher rate may be truncated to obtain the bitstream of a lower rate coding. Such an audio codec may utilize a codec designed purely for speech signals as the core layer or lowest bit rate coding.
An audio codec is designed to maintain high (perceptual) quality while improving the compression ratio. Thus it is common for the audio codec to adopt a multimode approach for encoding the input audio signal, in which a particular mode of coding is selected according to the channel configuration of the input audio signal.
An audio codec can be configured to operate with a multiple channel input audio signal, and in particular a bi-channel input audio signal. One such bi-channel configuration may be a stereo audio signal comprising two similar audio signals each having a different phase and sound pressure level. These differences may be attributed to the stereo signal being acquired by two omnidirectional microphones separated a reasonable distance apart. Another bi-channel configuration can be a binaural signal which is distinguished from the stereo signal by being acquired by two omnidirectional microphones with a relatively short separation. Typically the distance of separation when acquiring a binaural signal is of the order of a few centimeters to be commensurate with the distance between the right and left ears of the typical human head.